In the field of analog data transmission, one efficient data transmission technique is to utilize a TDM signal in which information corresponding with a plurality of sources is transmitted over a single data line. Data corresponding with each source is transmitted over the line in dedicated intervals which are generally regular in duration and sequenced. That is, at one particular point in time, data present on the line corresponds with only one of the sources. If the dedicated interval rate is sufficiently rapid, an apparency of continuous data transmission corresponding with each source is realized at the receiving end of the data line. In this regard, the TDM signal is de-multiplexed at the receiving end so as to separate the data into parallel channels, one corresponding with each source. De-multiplexing is generally performed in a synchronous switching operation.
In some systems, after de-multiplexing, a first series of signal conditioning steps is performed which operate on the parallel channel source data. Thereafter, a second series of steps is performed which, once again, require the signal to be in a TDM form. Re-multiplexing of the parallel channels is necessary to regain the TDM signal format required for the second series of steps. After the second series of signal conditioning steps, the signal is de-multiplexed a second time into parallel channels for completion of analog signal processing. The performance of each multiplexing/de-multiplexing iteration introduces switching noise into the resultant signal(s). As can be appreciated, such noise presents system design considerations and limitations.
Other limitations are also introduced by the performance of multiple de-multiplexing/multiplexing iterations. Specifically, each time either of these operations is performed, the overall parts count in the system is increased. Such an increase may significantly limit the reliability of the system and increase manufacturing costs. Moreover, the associated increase in signal line length resulting from the additional parts, along with their interconnections, may serve to couple still further noise into the system from the ambient environment, thereby reducing system performance.
The noted design considerations/limitations are of particular importance in medical instruments that determine pulse rate and blood oxygen saturation level via measurement of certain blood analytes such as, for example, the concentration (as a percentage of total hemoglobin) of oxyhemoglobin (O.sub.2 Hb), deoxyhemoglobin (RHb), carboxyhemoglobin (COHb) and methemoglobin (MetHb) of a patient. Such photoplethysmographic measurement instruments are configured to emit light of at least two different, predetermined wavelengths through a selected portion of a patient's anatomy (e.g., a finger tip). The analytes to be identified within the patient's blood must each have unique light absorbance characteristics for at least two of the emitted wavelengths. By measuring changes in intensity of the transmitted (the light exiting an absorber is referred to as transmitted) light from the patient's finger (or other suitable area of anatomy) at these wavelengths, each analyte may be determined. Thereafter, characteristics such as blood oxygen saturation may be determined based on these analytes. Other characteristics such as pulse rate may be determined based on certain components of the transmitted light signal which passes through the patient's anatomy. Specifically, the transmitted light includes a large DC component and a smaller AC or pulsatile component. By using the pulsatile component, the patient's pulse rate may be determined, since fluctuations in the pulsatile component are a function of arterioles pulsating with the patient's heart rate.
In one photoplethysmographic measurement system known as a pulse oximeter, at least two wavelengths of light may be emitted during dedicated, alternating intervals. The transmitted light from the selected body portion is detected by a light-sensitive element (e.g., a photodiode). The light-sensitive element then outputs a TDM signal that includes portions corresponding with each wavelength of the transmitted light. As will be appreciated, the photodiode is also sensitive to light which is present in the ambient environment. Consequently, the TDM output signal can include a corresponding ambient light component. Such component must be removed from the TDM signal for proper processing. For this purpose, at least one interval within a TDM signal is typically dedicated to measuring a component corresponding with only the detected ambient light.
For example in one known pulse oximeter, each emitted light level is immediately preceded by an ambient light interval which may also be referred to as a "dark time" interval. The system first de-multiplexes the TDM signal into parallel channels. Signal processing then proceeds wherein a first series of steps performs preliminary filtering. Immediately following the first series of steps, the parallel channels are re-multiplexed. Next, a second series of steps is performed in which the re-multiplexed signal facilitates subtraction of the dark time signal from the signal corresponding with each emitted light interval in a manner known in the art. Such subtraction process relies on a dark time interval immediately preceding each and every emitted light interval in a TDM format. Following the second series of steps, in which ambient light subtraction is accomplished, the TDM signal is de-multiplexed a second time into parallel channels prior to the completion of signal processing. Such multiple de-multiplexing/multiplexing raises the very noise introduction and cost concerns noted above.